Process of mercury casting



;other liquids, such as water for instance.

United States Patent 2,807,848 PROCESS OF MERCURY CASTING Charles A. Clements, Victoria, British Columbia, Canada, assignor to Mechanical Research Ltd., Victoria, British Columbia, Canada, a corporation of Victoria, British Columbia, Canada No Drawing. Application June 2, 1955, Serial No. 512,896

2 Claims. (Cl. 22-2165) This invention relates to mercury casting.

In the art of moulding metals the usual process has been to pour the metal, in its molten state, into a preformed mould usually of iron or steel. Such process, by reason of the need for rapid dissipation of heat, required that tolerances be wide dimensionally.v Recently, the mercury casting process has been introduced which allows for much closer tolerances and the casting of hard-to-cast metals such as stainless steels. The moulds in this process are subjected to sub-zero temperatures and never to applied heat as in ordinary foundry processes. The moulds used in this new art never at any time become receptacles for the molten metal; they are used solely as receptacles to turn a liquid into a solid at very low sub-zero temperatures.

In this new art of mercury casting, it has been found that inaccuracies build up on the surfaces of the master metal mould progressively with use, thereby affecting the dimensions or intended accuracy of the master mould.

For accuracy and stability in manufacture, and to withstand abuse in the continuing handling in the factory, these moulds have been made of hard metals such as steel.

As is known, the mercury in the liquid state is poured into a metal master mould, and the mould is lowered into a chamber having a temperature of about F. and is left therein until the mercury becomes solid as a core or pattern. After the mercury has so solidified, the mould is raised and then separated by disassembling, and withdrawn from the mercury pattern. The solid mercury pattern is then lowered into a ceramic slurry, which in turn is solidified to form a shell pattern to duplicate the original metal master mould. At room temperature, the mercury gradually drips from this shell as the mercury returns to its normal liquid state. This shell is then set into fine sand in the foundry core box into which molten metal is then poured to produce a casting to close dimensional accuracy.

The metal master mould is at a temperature not far removed from 100 F. after it has been opened for the solid mercury pattern to be left exposed and fully separated therefrom. For continuity of production, this metal master mould is again closed and liquid mercury is poured into it, to repeat the operation, as rapidly as possible, to reduce costs of the process which are high under the best of conditions.

Mercury that has returned to liquid from a solid state is used over and over again, and during the course of repetitive use picks up deleterious and foreign matter.

The metal master mould is subject to rapid temperature changes within a range of over 100 F., with the result that oxidation is formed on the surfaces of the mould.

As the mercury freezes, it expands in volume as do The internal pressure set up within the liquid mercury as it goes into the solid state forces the foreign matter to the surfaces, which foreign matter tends to bond with the metal surfaces of the mould, which have been roughened from previous smoothness by oxidation. The result is that the surfaces which should be smooth are irregular or rough, necessitating constant inspection and much hand work and polishing which causes wear on the mould and therefore loss of dimensional accuracy.

It has been the practice to impart a high degree of smoothness to the metal master mould and endeavour to retain such smoothness as far as practicable during the manufacturing processes, and until the mould itself has to be reworked or replaced. This is highly costly.

Thus far, no method has been found to prevent thev unusual and disturbing oxidation of the metal surfaces:

of the mould found to take place at excessively low temperatures and accelerated by rapid changes in temperature specially when in contact with mercury.

Moulds of aluminum alloys could be made much more cheaply and more quickly than can moulds of steel or other metal alloys, and to practical equal dimensional accuracy. 'However, the aluminum easily scratches with handling and has great aflinity for foreign elementsand easily oxidizes, and therefore does not offer a practical solution.

I have now discovered that a practical solution is to use a mould of aluminum, or substantially. aluminum, which has been anodized, so that all exposed surfaces, including the mercury-engaging surfaces, are covered with an integral thickness of aluminum oxide. The anodized thickness of aluminum itself is an oxide of aluminum, being a replacement of the original surface by its own considerably harder oxide, without dimensional variation. As the mould is thus covered with a hard surface, it will withstand rough and careless handling without scratching. The mould is considerably lighter and less costly than the usual steel moulds in current use.

The aluminum mould may be made by any of the standard methods, such as by casting, machining or forging, before being submitted to the electrolytic anodizing process.

The cold applied through the body of the aluminum mould absorbs heat from liquid mercury at a rate of about twice that occurring when the conventional steel master mould is used, thus materially increasing the output. The naturally high polished surfaces obtained by the anodic treatment will not corrode by oxidation, being already oxidized. The anodized surfaces do not tend to bond themselves to any impurities in the mercury which come into contact therewith, so that these impurities are free to rise to the open top of the mould, and do so rise and tend to overflow since they have a much lighter den sity than the mercury.

Thus, the mould remains relatively clean, and requires only occasional wiping of its surfaces with liquid Silicone for continuing use. Silicone is a recently coined word designating a synthetic oil commonly used in'the fairly recent art of mercury casting. This synthetic oil is procurable on the market as Silicone.

As the value of the anodized mould has a direct relationship to the thickness of the hard aluminum oxide surfaces, it follows that the preferred treatment should provide for a minimum of lapsed time and a minimum voltage to be used. Experience has shown that a minimum coating is provided with a voltage of not less than 45 7 volts applied for part of an over-all period of not less than one hour.

As a check in practice, the mould is anodized in the electrolyte to which has been added a coloured dye, preferably organic in nature, the dye becoming integral with the aluminum oxide as it is formed. The mould, when removed from the electrolytic bath, washed with cold water, dried and rubbed with a damp cloth, preferably a cloth moistened with Silicone, will not produce any Patented Oct. 1, 1957 loss of colour from the anodized surface if the surface has been suflieiently anodized. If loss of colour takes place, then the mould must be replaced into the electrolyte and subjected to a voltage of 50 volts for ten minutes, and again colour tested. When the colour retention is satisfactory, the mould must then be immediately plunged into hot or boiling water to seal the pores.

What I claim is:

1. The method of making a casting pattern of frozen mercury, comprising forming a mold with a die cavity having the shape of the casting pattern, with at least one complementary mold member of said mold being being formed of metal consisting principally of aluminum, providing an electrolytic anodizing bath containing a coloring dye, subjecting all exterior surfaces of the aluminum mold member to electrolytic anodizing treatment in said bath for a sufficient time and at suflicient voltage for producing on the entire exterior surface of said mold member a substantially continuous layer of aluminum oxide coating of thickness determined by attenuation of the anodic process, repeatedly photometrically testing the density of color produced as integral with the coating repeating the anodizing process until the layer of oxide of sufficient depth has been uniformly attained, sealing the aluminum oxide pores with boiling water, thereafter assembling the so-treated aluminum mold member with the other complementary members to provide said mold cavity, thereafter filling said mold cavity with liquid mercury, and cooling the liquid mercury filling the mold cavity until it solidifies into a rigid casting pattern, and thereafter removing the frozen mercury pattern from said die cavity.

2. The method of making a casting patternv of frozen mercury, comprising forming a mold with a die cavity having the shape of the casting pattern, with all complementary mold members of said mold being formed of metal consisting principally of aluminum, providing an electrolytic anodizing bath containing a coloring dye, subjecting all exterior surfaces of said mold members to electrolytic anodizing treatment in said bath for a suflicient time and at sufiicient voltage for producing on the entire exterior surface of each of said mold members a substantially continuous layer of aluminum oxide coating of 10 thickness determined by attenuation of the anodic process, repeatedly photometrically testing the density of color produced as integral with the coating repeating the anodizing process until the layer of oxide of sufiicient depth has been uniformly attained, sealing the aluminum 15 oxide pores with boiling Water, thereafter assembling the so-treated complementary mold members to provide said mold cavity, thereafter filling said mold cavity with liquid mercury and cooling the liquid mercury filling the mold cavity until it solidifies into a rigid casting pattern, 20 and thereafter removing the frozen mercury pattern from said die cavity.

References Cited in the file of this patent UNITED STATES PATENTS 2,023,645 Newton et al. Dec. 10, 1935 2,400,831 Kohl May 21, 1946 FOREIGN PATENTS 30 543,577 Great Britain Mar. 4, 1942 OTHER REFERENCES Rawson L. Wood et al.: Investment Castings for Engineers, pages 101-117, 1st paragraph, page 104 relied upon (1952). Copy in Div. 3. 

1. THE METHOD OF MAKING A CASTING PATTERN OF FROZEN MERCURY, COMPRISING FORMING A MOLD WITH A DIE CAVITY HAVING THE SHAPE OF THE CASTING PATTERN, WITH AT LEAST ONE COMPLEMENTARY MOLD MEMBER OF SAID MOLD BEING BEING FORMED OF METAL CONSISTING PRINCIPALLY OF ALUMINUM, PROVIDING AN ELECTROLYTIC ANODIZING BATH CONTAINING A COLORING DYE, SUBJECTING ALL EXTERIOR SURFACES OF THE ALUMINUM MOLD MEMBER TO ELECTROLYTIC ANODIZING TREATMENT IN SAID BATH FOR A SUFFICIENT TIME AND AT SUFFICIENT VOLTAGE FOR PRODUCING ON THE ENTIRE EXTERIOR SURFACE OF SAID MOLD MEMBER A SUBSTANTIALLY CONTINUOUS LAYER OF ALUMINUM OXIDE COATING OF THICKNESS DETERMINED BY ATTENUATION OF THE ANODIC PROCESS, REPEATEDLY PHOTOMETRICALLY TESTING THE DENSITY OF COLOR PRODUCED AS INTEGRAL WITH THE COATING REPEATING THE ANODIZING PROCESS UNTIL THE LAYER OF OXIDE OF SUFFICIENT DEPTH HAS BEEN UNIFORMLY ATTAINED, SEALING THE ALUMINUM OXIDE PORES WITH BOILING WATER, THEREAFTER ASSEMBLING THE SO-TREATED ALUMINUM MOLD MEMBER WITH THE OTHER COMPLEMENTARY MEMBERS TO PROVIDE SAID MOLD CAVITY, THEREAFTER FILLING SAID MOLD CAVITY WITH LIQUID MERCURY, AND COOLING THE LIQUID MERCURY FILLING THE MOLD CAVIAFTER REMOVING THE FROZEN MERCURY PATTERN FROM SAID DIE CAVITY. 